The invention relates to an apparatus for detecting and locating a radioactive source emitting gamma rays. It also relates to the use of the apparatus.
In the rest of the description, the detection apparatus of the invention applies more particularly, but not exclusively, to the detection of lymph nodes, some of which by fixing an in situ injected radioactive substance are capable of characterizing the extension of a tumor, especially in the case of breast cancer or skin cancer (melanoma).
However, other applications of the apparatus may be envisaged, such as the search for lost radioactive sources, the drilling of a material based on information available on its other face and, more widely, the detection and location of an almost point-like radioactive source through a medium that does not completely absorb the gamma rays that it emits, without this being restrictive.
Primary cancers firstly exhibit a local stage followed somewhat later by secondary sites. The extension process starts even before the clinical manifestations of dissemination are detectable. The first anatomical structures attacked outside the primary tumor are generally the lymph nodes. Thus, one cancer treatment consists in ablating the lymph nodes of the region in question. The purpose of such an operation is not only to remove possible secondary sites but also to diagnose possible metastasic dissemination, thus making it possible to propose the indication of a complementary medical treatment.
In the Particular case of breast cancer, the surgical treatment consists, apart from removing the primary tumor, in also removing the lymph nodes located in the axillary hollow, this operation being called xe2x80x9clymph node curagexe2x80x9d, and then in carrying out a histological analysis of the lymph nodes removed.
Such an operation is a major source of functional sequela for the patient. First of all, the lymphatic engorgement resulting from removal of the lymphatic system draining the major part of the upper limb generates a pathology called lymphoedema or xe2x80x9cfat armxe2x80x9d syndrome causing as much a functional impediment as unsightliness. Furthermore, it turns out that during curage the branches of the first intercostal nerves present in the axillary region are cut, thus leading to the appearance of sensitive disorders. Motor disorders of the shoulder also appear.
Furthermore, and above all, it turns out -hat the lymph nodes removed during axillary curage do not always prove, by histological analysis, to be invaded. It should be noted, in particular, that in the case of tumors less than or equal to 10 mm in size, only 6% of the lymph nodes removed are invaded. In other words, 95% of curages carried out are in fact unnecessary.
This is the reason why a number of techniques have been developed to try to detect preoperatively and/or preoperatively, and then to remove, only the lymph node or nodes likely to be attacked during the secondary extension of a primary tumor.
Thus, in a case of the treatment of a melanoma, it has been found that the first, relay node draining the melanoma (called the sentinel node) is capable of fixing a dye, particularly patent blue.
It has been demonstrated that the absence of invasion of the sentinel node corresponds in almost 100% of cases to the absence of invasion of the other nodes. It therefore becomes possible, by carrying out an extemporaneous analysis of the sentinel node, once identified and then removed, to decide during intervention whether or not to continue the lymph node curage.
However, according to this technique, since identifying the sentinel node is strictly visual, it is necessary to dissect the tissues in the region where it is assumed the sentinel node is. However, since the sentinel node has no perfectly defined location from the anatomical standpoint, this search is not simple and may prove to be lengthy and relatively impairing.
To solve this problem, it has been proposed to replace the dye with a radioactive substance. Detection of the sentinel node is then performed by one of two techniques, respectively:
either by scintigraphic imaging (lymphoscinti-graphy) using a scintillation camera;
or using a peroperative detection probe.
It turns out that scintigraphic imaging of lymph nodes, although validated in the display of nodes in non-cancerous pathology, is poorly suited to the specific detection of the sentinel node.
This is because, owing to its imposing weight (more than 1 tonne), the imaging apparatus is not transportable, so that the examination has to be performed preoperatively and elsewhere than in the room where said operation is being carried out, thus complicating the planning organization for the various parties involved.
Moreover, and above all, locating is performed by means of skin marking guided by a radioactive source brought into correspondence with the hyperfixing area on the image. Consequently, this mark is necessarily imprecise since;
firstly, it depends on how the head of the camera is angled;
secondly, it is often difficult to mark the skin accurately in a soft and not easily accessible region;
and finally, the surgeon cannot check in real time the well-foundedness of the skin mark.
At the same time, the angle of incidence of the incision may be different from that of the imaging, resulting in a location error which is all the greater the deeper the node.
In other words, the scintillation camera allows only coarse pre-locating of the position of the node during the preoperative stage.
On the other hand, the detection probe is a small piece of equipment, which can therefore be manipulated manually. This probe detects the gamma photons emitted by a radioactive source and allows audible location of said source placed within the beam angle of its collimator.
Technically, the detection probe includes a collimator consisting of a single hole, a scintillating crystal, a photomultiplier with a single anode, and associated electronic means for assessing, by a displayed numerical value or an audible signal, the intensity of the signal detected. However, this detection probe can only be used to assess the amount of radioactivity present in the field of view of its collimator, without being capable of specifying the precise direction in which the radioactive source lies.
In addition, it is very tricky to locate the node because of the very restricted angle of observation of the system and the large number of degrees of freedom in positioning. Moreover, because of the variability in the angle of incidence, and therefore in the thickness of the tissues through which the radiation emanating from the node passes, together with the inconstancy of the positioning, since the probe is hand-held, locating the nodes requites a great deal of time and sometimes proves to be fruitless.
In other words, the problem that the invention aims to solve is to provide an apparatus which allows preoperative and above all peroperative detection and precise location of a radioactive source, thus making it possible, for example in the case of the detection of lymph nodes, to avoid any unnecessary impairment in view of or during the surgical operation.
To solve this problem, the invention proposes an apparatus for detecting and locating a radioactive source emitting gamma rays.
This apparatus is characterized in that it comprises:
first means for determining the direction of the gamma-ray-emitting source with respect to the center of the detector;
a second means capable of pin-pointing the radioactive emission source.
In other words, the invention consists of a detection system making it possible during a surgical intervention to identify and precisely locate organs or tissues, and especially lymph nodes fixing a radioactive source emitting gamma rays.
Contrary to the technique of imaging using a scintillation camera, the objective is not to form a faithful image of the tissue or organ fixing the radicactive source, but to identify in which direction with respect to the center of the detector said source lies, so as to control the movement of the detector for the purpose of bringing this source opposite the central part of the detection area.
To achieve this objective, said first means comprise:
gamma-ray sensor means;
a plurality of means for measuring the gamma-ray flux;
means for analyzing the gamma-ray flux which are capable of determining the direction of the radioactive source.
In practice, the means for sensing the gamma rays consist of a collimator and a scintillating crystal capable of emitting a light signal under the effect of a gamma ray.
To sense the gamma rays emitted by the source, whatever their direction, the collimator has a central area comprising a plurality of mutually parallel channels which are perpendicular to the surface of said collimator.
Advantageously, the number of parallel channels is four.
According to another embodiment, the collimator furthermore has a peripheral area comprising a plurality of divergent channels making, with the channels of the central area, an angle which increases with their distance from the central area.
The specific structure of the collimator therefore makes it possible, because of its peripheral part, to detect the region of a possibly remote source without worrying about the precision in the location and, of a central part, to have a high special resolution so that the subsequent step of locking onto the direction of the tissue or organ fixing the radioactive source is precise.
Finally, It is conceivable to use an asymmetric collimator consisting of truncated divergent channels.
As already mentioned, the collimator is attached on its rear face to a scintillating crystal capable of emitting a light signal under the effect of gamma rays. In practice, however, this crystal consists of a thallium-activated cesium iodide or sodium iodide crystal. It has a thickness sufficient to stop a high proportion of the gamma photons to be detected. In practice, the thickness of the crystal is between 5 and 10 mm for detecting photons having an energy of 140 keV emitted by 99 m-technetium. The scintillating crystal has an overall round shape whose diameter depends on the photomultiplier used, this being described later.
As already mentioned, the precision in locating the impacts received by the peripheral part does not matter. All that it is required to know is whether the detector has to be moved more in one direction than in another.
To measure the flux of gamma rays picked up by the collimator, the apparatus of the invention includes a plurality of measuring means consisting of one or more single-anode photomultipliers capable of receiving the light signal emitted by the sensor means.
Advantageously, the number of single-anode photomultipliers is our.
In another advantageous embodiment of the apparatus, the means for measuring the gamma-ray flux consist of a multi-anode photomultiplier. The signal output by each anode would then be regarded as being equivalent to that output by the anode of a single-anode photomultiplier.
To determine precisely the direction of the radioactive source, the first means comprise, as already mentioned, means for analyzing the gamma-ray flux measured by the measuring means. The purpose of this analysis is to determine in what direction with respect to the center of the field of detection the radioactive source lies. The analysis indicates that the source is either offset in one particular direction or lies on the axis of the detector. The analysis is carried out by comparing the signals output by the measuring means and especially the photo-multipliers. This analysis uses electronic means to determine among the signals generated by each of the photomultipliers, or by a group of photomultipliers, which has the highest intensity.
In an advantageous embodiment, using the Anger camera principle, the signals output by the anode of each single-anode photomultiplier or each anode of the multi-anode photomultiplier are combined in order to form four signals coded in X and Y with respect to the center of the photomultiplier, these being denoted Xxe2x88x92, X+, Yxe2x88x92, Yxe2x88x92, and the combination of which provides the X and Y coordinates of a given point. Thus the detected intensities are stored in four registers, for example North, South, East and West, according to the positivity of X and Y.
When only the data corresponding to the signals output by the central area of the crystal are to be processed, only the signals whose absolute value is less than a prefixed threshold are stored.
In an advantageous embodiment, the means for analyzing the measured flux are capable of taking into account all or only some of the intensities output by the measuring means, depending on the extent to which the intensities are almost uniform.
In other words, the data processing is carried out firstly on the basis of the signals output by the entire sensitive area of the crystal, making it possible to rapidly move the center of the detector closer to the source lying within the scan field and then, only from the central part of the crystal, making it possible to center the detector, more precisely, but more slowly, on the hot spot, whatever the sources lying in the rest of the scan field.
To enable the source of radioactive emission to be pinpointed, when the direction of gamma-ray emission has been detected and the emission of a signal has taken place, said second means of the detection apparatus of the invention comprise:
mechanical means capable of imparting a movement to the apparatus so as to move its central axis closer to the radioactive source emitting gamma rays;
means capable of defining the central axis of the apparatus.
In practice the mechanical means capable of imparting a movement to the apparatus comply with movement instructions as long as the analyzing means indicate that the central axis of the detector is not in correspondence with the source.
Moreover, the means capable of defining the central axis of the apparatus are in the form of a light ray.
According to an advantageous embodiment, said means are in the form of at least two coherent light sources generating narrow light beams, the intersection of which corresponds to the perpendicular at the center of the sensitive area of the collimator (center of the apparatus).
In one particular embodiment, the light source is positioned in such a way that the edge of the beam corresponds to a boundary of the region scanned by the collimator.
As already mentioned, the apparatus of the invention can be used to detect a human or animal tissue or organ fixing a gamma-ray emitting radioactive source.
In the case of skin cancer or breast cancer, the apparatus will be more particularly used for detecting lymph nodes which, by fixing a gamma-ray emitting radioactive source, testify to the secondary extension of the primary tumor.
The apparatus can also be used for the preoperative detection of an anomaly, such as for example breast microcalcifications, which microcalcifications will have been previously labelled by the in situ injection of a radioactive substance under radiographic or echographlc imaging control.
More generally, the apparatus of the invention may be used for detecting and locating a localized radioactive source through a medium which does not completely absorb he gamma rays that it emits.
Finally, the invention relates to a method of detecting a radioactive source emitting gamma rays fixed by a human or animal tissue or organ in which:
a radioactive substance emitting gamma rays is injected into the body;
the direction of the gamma-ray emitter is determined; characterized in that the direct-on determined is defined so as to pinpoint the source of radioactive emission.